Gyro stabilizer for helicopter



June 5, 1962 w. GERSTENBERGER ETAL I 3,037,722

GYRO STABILIZEIR FOR HELICOPTER Originai Filed Aug. 4, 1953 12Sheets-Sheet 1 INVENTORS H.T. JENSEN v, /3 W ATTORNEY WALTERGERSTENBERGER June 5, 1962- w. GERSTENBERGER ET AL 3,037,722

GYRO STABILIZER FOR HELICOPTER l2 Sheets-Sheet 2 INVENTORS WALTERGERSTENBERGER BY H. T. JENSEN ATTORNEY Original Filed Aug. 4, 1953June'5, 1962 w. GERSTENBERGER ETAL 3,037,722

GYRO STABILIZER FOR HELICOPTER Original Filed Aug. 4, 1953 12Sheets-Sheet 5 ATTORNE Y June 5, 1962 w. GERSTENBERGER ETAL GYROSTABILIZER FOR HELICOPTER 12 SheetsSheet 4 Original Filed Aug. 4, 1953INVENTORS \l m! m WALTER GERSTEN BERGER BY H.T. JENSEN ATTORNEY June 5,1962 w. GERSTENBERGE'R ETAL 7,

GYRO STABILIZER .FOR HELICOPTER Original Filed Aug. 4, 1953 12Sheets-Sheet 5' INVENTORS WALTER GE RSTENBERGE R [88 BY H.T. JENSENATTORNEY June 5, 1962. w. GERSTENBERGER ETAL 7,

GYRO STABILIZER FOR HELICOPTER Qriginal Filed Aug. 4, 195$ 12Sheets-Sheet 7 71/5 IN; III/IIIIIIIIII. \IIA KAI POSITION HEADING l7?GYRO SELECTOR I 236 I I v I l RATE L L AMPLIFIER GYRO RATE J C GEN. I345 Fig. 13

- 144 I66 RATE FOLLOW n CANOELER NULLER GEN. WUPTRANSMINTOR 149/ 1 /7o-27/ 332 AMPLIFIER Q I WALTER GERSTErII'EI EER G R RATE POSITIONALTIMETER" GYRO GYRO BY H.T. JENSEN ATTORNEY June 5, 1962 w.GERSTENBERGER ET AL 3,037,722

GYRO STABILIZER FOR HELICOPTER l2 Sheets-Sheet 8 Original Filed Aug. 4,1955 mwijmfd v Om o 202.501

INVENTORS WALTER GERSTENBERGER H.T. JENSEN 2 a 7 ATTORNEY June 5, 1962W. GERSTENBERGER ET AL GYRO STABILIZER FOR HELICOPTER Original FiledAug. 4, 1953 12 Sheets-Sheet 9 'IIIIIIIIIIIII// l NULLING POTENTIOMETERSENSITIVITY ADJUSTMENT k POSITION HEADING I66 GYRO SELECTOR C AMPUHERgggg RATE GEN. FOLLOW UP L L POTENTIOMETER W5 BALANCING POTENTIOMETER:Fig. 15

OTHER SENSING ELEMENTS INVENTORS WALTER GERSTENBERGER BY H. T. JENSEN 717.

ATTORNEY June 5, 1962 w. GERSTENBERGER ET AL 3,037,722

GYRO STABILIZER FOR HELICOPTER Original Filed Aug. 4, 1953 12Sheets-Sheet l0 STEADY STATE HOVERING NULLED STEADY STATE FORWARD FLIGHTlTl zs PILOTS STICK T0 27/ b SWASHPLATE' CANCELER POTENTIOMETER if 272SENSITIVITY ADJUSTMENT 276 27 8 3 I 0 THE'R SENSING ELEMENTS BALANCE RINVENTORS WALTER GERSTENBERGER EXITATION ATTORNEY 12 Sheets-Sheet 11LATERAL OTHERSENSING ELEMENTS W. GERSTENBERGER ET AL GYRO STABILIZER FORHELICOPTER June 5, 1962 Original Filed Aug. 4

EXITATION INVENTORS WALTER GERSTENBERGER BY H.T. JENSEN 7 flww iuATTORNEY June 5, 1962 Original Filed Aug. 4, 1955 GYRO STABILIZER W.GERSTENBERGER ET AL FOR HELICOPTER l2 Sheets-Sheet 12 VERT. PITCH GYROVERT. STICK PROPORTION sENs. NuLL. eYRofslg ELER I I I I I LONG. POSIT.PICK- OFF i O l 1 l 0N J27,

FOLLOW-UP SENS- cANcELER PITCH FOLLOW-UP COMPASS SENSITIVITY RATE GYROALT.TRAN$.

f-lal FOLLOW-UP 5 TRANS.

w 344 VELOCITY SENS ZERO READING GEN. 346 HEADING SELECTOR \l79 RATEGYRO PITCH BALANcE coN'rRoL AMPLIFIER RATE Sula /F6LLOW-UP TRANS.

zw" g MOTOR f 1 /79 2 Y YAW AMPLIFIER aso :Flg. 213 INVENTORS WALTERGERSTENBERGER BY H. T. JENSEN ATTORNEY @MOTOR atent Office 3,937,722Patented June 5, 1962 3,037,722 GYRO STABILIZER FOR HELICOPTER WalterGerstenberger and Harry T. Jensen, Milford, Conn, assignors to UnitedAircraft Corporation, East Hartford, Conn., a corporation of DelawareContinuation of application Ser. No. 372,265, Aug. 4, 1953. Thisapplication June 12, 1958, Ser. No. 741,531

32 Claims. (Cl. 24417.13)

This application is a continuation of our copending application SerialNo. 372,265, filed August 4, 1953, now abandoned, and relates to anautopilot for aircraft, such as helicopters or airplanes and has as ageneral object the provision of an improved autopilot capable ofcontrolling pitch angle, roll angle, and yaw angle as well as thelateral, longitudinal and vertical displacement of the aircraft. Whilethe invention is shown applied to a helicopter, many of the novelfeatures thereof are equally well adapted to autopilot systems for othertypes of aircraft.

Some of the objects of the invention are to provide an autopilot inwhich the primary servomotor controls of the aircraft are utilized asactuating members for the autopilot; to provide a differential systemwhich will enable the pilot signal and/ or the autopilot signal tocontrol the aircraft without turning off the autopilot; to provide asmall electric sub-servomotor in the primary hydraulic control system toproduce a differential control correction at the output power level ofthe sub-servomotor for a stick fixed autopilot system; to provide meansto lock out the differential input of the sub-servomotor in the event ofa primary hydraulic failure; to utilize means to produce in the samesystem a combination of differential control correction for smallsignals and integrated feedback corrections required in the yaw systemby opening the feedback loop in a combined electrohydraulic servo; toinsert a voltage into the servosys tem which is proportional to stickdisplacement to offset in the differential system the bucking effect ofthe autopilot when the pilot moves the control stick to achieve a newflight condition; to control the reference level of any of the controlmovements of the aircraft so that the pilot can engage the autopilot inany flight attitude and have the autopilot hold the ship in thatattitude while at the same time enabling the pilot to alter thereference setting to which the craft is slaved without disengaging theautopilot; and to provide an autopilot which is capable of controlling ahelicopter to maintain the same fixed in space.

These and other objects and advantages of the invention will becomeevident from the following detailed description of a preferredembodiment of the invention.

In these drawings:

FIG. 1 is a side elevation of a helicopter equipped with the autopilotof this invention;

FIG. 2 is a view showing the pilots controls for the helicopter of FIG.1;

FIG. 3 is a diagrammatic view of the rotor head of the helicopter ofFIG. 1;

FIG. 4 is an enlarged detail of a portion of the control mechanism ofFIG. 2;

FIG. 5 is a detail front view of a portion of FIG. 4;

FIG. 6 is a still further enlarged sectional view of a portion of FIG.4;

FIG. 7 is a detail view partly in section of one of the primaryhydraulic servomotors;

FIG. 8 is a detail sectional view of the rate generator, sub-servomotorand follow-up transformer unit;

FIG. 9 is a schematic showing of the tail rotor pitch I control;

FIG. 9a is a detailed view on an enlarged scale of a part of FIG. 9;

FIG. 10 is a view showing the mixing linkage of FIG. 9 in greaterdetail;

FIG. 11 is a block diagram showing the relationship of the components ofthe autopilot system;

FIG. 12 is a diagrammatic view of the yaw control system;

FIG. 13 is a diagrammatic view of the pitch control system;

FIG. 14 is a diagrammatic view of the roll control system;

FIG. 15 is a modified diagrammatic view of a general control systemincorporating a pilot feel into the controls;

FIG. 16 is a diagrammatic view showing the nulling system;

FIG. 17 is a diagrammatic view showing the stick canceler;

FIG. 18 is a diagrammatic view illustrating the stick canceler inhovering;

FIG. 19 is a diagrammatic view illustrating the stick canceler inforward flight;

FIG. 20 is a side view of the device for controlling hovering over afixed point;

FIG. 21 is a diagrammatic view illustrating the lateral control for thedevice of FIG. 20;

FIG. 22 is a diagrammatic view illustrating the longitudinal control forthe device of FIG. 20;

FIG. 23 is a wiring diagram of the pitch control system; and

FIG. 24 is a wiring diagram of the yaw control system.

As herein shown, the helicopter embodying the invention includes anelongated fuselage 10 having a main cargo or passenger compartment 12directly beneath a main sustaining rotor generally indicated at 14 andhaving an antitorque tail rotor 16. An engine 18 is provided in a nosecompartment 20 and a pilot compartment 22 is provided above the enginecompartment in which are located the usual collective pitch controllevers 24 and the cyclic pitch control levers 26 for the pilot andcopilot. Directional control pedals 27 are connected to control thepitch of the directional control rotor 16 in the usual manner. Theblades 28 of the main rotor are of the articulated type which are freeto move about flapping hinges 28a and drag hinges 28b (FIG. 3) and tomove about their own horizontal axes 280 to vary their pitch in flight.Each blade is provided with the usual blade horn 30 connected by a pitchcontrol rod 32 with the rotating part 34 of the usual swash plate whichis universally mounted at 36 (FIG. 4) on an upright drive shaft 38. Thenon-rotating part 40 of the swash plate is mounted on the part 34 byusual antifriction bearings 42 and is connected to the pilots controlsas will hereinafter be described.

The autopilot of this invention, as illustrated herein, is intended toprovide the following controls of a helicopter:

(1) Fuselage pitch angle and rate of fuselage pitch angle changemeasured by gyroscopic means to correct the longitudinal cyclic pitch.

(2) Fuselage roll angle and rate of fuselage roll angle change measuredby gyroscopic means to correct lateral cyclic pitch.

(3) Fuselage azimuth angle and rate of fuselage yaw measured bygyroscopic means to vary tail rotor pitch.

(4) Vertical error sensed by barometric altimeter to correct collectivepitch.

(5) Bodily displacement laterally controlled by means for measuring theangular displacement of the fuselage relative to a point on the groundto correct the lateral cyclic pitch.

(6) Bodily displacement longitudinally controlled by means for measuringthe angular displacement of the 3 fuselage relative to said point tocorrect the longitudinal cyclic pitch.

Because of the inherent stability problems associated with a helicopterof this type, it is desired to have an autopilot which can be leftengaged during all flight conditions including all maneuvers.Consequently it is advantageous to eliminate the necessity of two meansof flight control, i.e. the conventional pilot stick with the autopilotofif and the maneuvering stick with the autopilot engaged. The presentsystem enables the pilot to use the conventional flight controls bothwith and without the autopilot engaged. Fundamentally then, the pilotmust be able to overpower the autopilot and this may be done either by(l) ofl-setting any control correction made by the autopilot(differential control system giving a fixed stick when autopilot aloneis controlling helicopter) or (2) overcoming the fixed capabilities ofthe autopilot (necessitating a free stick when the autopilot is inoperation).

The present autopilot is basically of the fixed stick type but certainmodifications are required to allow the autopilot to function withlimited authority so that the amount of control necessary for the pilotto override the autopilot will not be excessive. In pitch and roll asocalled canceler is used which essentially alters the referencefuselage angle when the stick is moved by the pilot. This then makes thecyclic pitch stick essentially a fuselage angle control in the staticsense.

In yaw, however, tail rotor pedal angle cannot be and is not made anazimuth angle control in the sense that a steady state pedal position isassociated with a compass heading. Consequently other means are requiredto incorporate the difierential system in the directional control. Asmall range of dilferential control is allowed, but when steady stateerrors, arising from large change of rotor torque for instance, demandlarge tail rotor corrections the autopilot is able to exert a force onthe pilots pedals which if not overcome by the pilot will cause thepedals to move farther, correcting tail rotor blade angle until thefuselage has been driven around to the desired azimuth position. Thisbecomes, with large errors, essentially an integrating feedback.

Considering first the conventional pilot controls for the helicopter itwill be noted from FIG. 3 that the nonrotating element 40 of the swashplate has four control bosses 44 equally spaced about its periphery. Tothree of these the usual controls from the pilots sticks are pivotallyattached, and these control bosses are further identified in FIG. 2 as44a, 44b and 440 to which the pilots controls for imparting forward,left and right movements to the ship are attached. The fourth point 44is connected to a usual scissors (not shown) which connects the swashplate element 40 to fixed structure of the helicopter so that it is freeto tilt but cannot rotate.

The dual pilots controls are shown most clearly in FIG. 2. Movement ofeither cyclic pitch stick 26 in a fore and aft direction about itsuniversal support 46 will rotate shaft 48 and the arm 50 carried therebyto reciprocate thrust rods 52, 54 and 56, thereby to impart movement tothe swash plate at boss 44a through the combined hydraulic andelectrical servomotor unit 58 or through the manual link 60 alone, asdescribed below. Lateral movement of sticks 26 are effected in unison bya tie rod 62. A rod 64 connects rod 62 to a rocking lever 66 and througha thrust rod 68 connects the sticks to a double bellcrank 70 which hasits opposite arms connected to thrust rods 72 and 74 so that the latterare differentially operated upon lateral movements of the sticks. Thereciprocatory movements of rods 72 and 74 are transmitted throughsuitable bellcranks 75 and rods 76 and 78 to rods 56 and links 60associated with the units 58 and to bosses 44c and 4417 which providefor lateral tilting of the tip path plane of the rotor blades.

Rotation of rod 80 by the collective pitch sticks 24 exerts a thrust onthe two rods 82 which effects movement of a pair of bellcranks 84 abouttheir fixed pivots 86 to raise and lower a shaft 88 on which the threebellcranks A. 75 of the cyclic pitch linkage are pivotally carried. Thisimparts an equal thrust to the rods 76, 54 and 78 which in turn impartsan equal thrust to the rods 56 and through links 60 to all three bosses44a, 44b and 440 on the relatively stationary swash plate 40.

One of the three electro hydraulic servomotor units 58 is shown in FIGS.4 to 6. It will be noted in FIG. 4 that each of the rods 56 is pivotedat 92 to one end of a link 94 the other end of which is pivoted at 96 toa bracket 97 carried by a pylon $8 of the helicopter. Link 60 is alsopivoted at its lower end to link 94 at 100. At its upper end link 60 ispivoted to the boss 44 by a pin 102 with a lost motion connection whichis shown most clearly in FIG. 6. Here it will be seen that link 60 and alink 104, the bifurcated upper ends of which lie adjacent, have enlargedapertures in their upper ends in which short bushings 106 are fitted toreceive loosely a bushing 108 which is a close fit on the pin 102. Thelink 104 which with link 60 has the lost motion connection relative topin 102 also has a pivotal connection 110 at its lower end with a lever112 which is pivoted intermediate its ends at a point 114 to a pilotvalve 116 reciprocable in valve casing 117 carried by the hydraulicservomotor. This pilot valve (FIG. 7) has upper and lower lands 118 and120 which meter fluid under pressure entering at 122 selectively topassages 124 and 126 which communicate with the chambers 128 and 130above and below a power piston 132. Piston 132 is mounted on a pistonrod 133 in a cooperating cylinder 134. The usual internal passagesincluding the axial passage 136 in the pilot valve 116 are provided, bywhich fluid on the low pressure side of the piston 132 is allowed tovent through a duct 138. The lower end of piston rod 133 is pivoted at140 to bracket 97. The cylinder 134 is pivoted at its upper bifurcatedend on bushings 108 and hence on pin 102 as shown in FIG. 6. The innerrace of a ball bearing 141 is mounted on pin 102 between the furcationsat the upper end of cylinder 134. Its outer race is carried by a boss 44of the relatively stationary part 40 of the swash plate.

From the above it will be evident that movement of the pilots controlsto move any one of the rods 56 results in an initial movement of links60 and 104 without moving the swash plate due to the so-called sloppyconnection of these links with pin 102 afforded by the enlarged bushing106. The effect of this initial movement is to rock lever 112 about itsleft hand end (FIG. 4) where it is pivoted at 142 on a sub-servomotorunit 144, hereinafter described. This movement of 112 moves valve 116 toadmit fluid under pressure to the power servomotor and causes thecylinder 134 to move the lug 44 and the swash plate- It will be evidentthat if rod 56 is moved up lever 112 will move counterclockwise about142 and fluid will be admitted to cylinder 134 by the pilot valve abovethe piston 132. Cylinder 134 will be moved upward and in so doing willcarry the pilot valve casing 117 with it, since the pilot valve casingis integral with cylinder 134, thus providing a follow-up for themovement of the pilot valve originated by the control stick.

As shown in FIG. 7, communication between passages 124 and 126 isprovided for by a valve 146. This valve, shown schematically in 'FIG.13, is kept closed by pressure of the hydraulic system which compressesa spring 147. This valve when opened by failure of the pressureautomatically establishes free communication between the chambers onopposite sides of the primary power piston 132.

Automatic means is likewise provided for simultaneously eliminating thelost motion above described between bushings 106 and 108 whenever thepressure in the hydraulic system fails. This mechanism which isconveniently termed a slop eliminator is illustrated most clearly inFIG. 6 and includes a pair of diametrically opposed cylinders 148,closed at their remote ends, and carried by link 60. These cylindershouse a pair of compression springs .150 which bear against the closedends of the cylinders and against piston elements 152. The confrontingpiston heads under the action of the springs normally abut the annularstop 154 and provide a common chamber 156 into which hydraulic fluidunder pressure is introduced from the hydraulic system through passage158. Passage 158 may communicate with the high pressure side of thehydraulic system wherever convenient, for example, as here shown, withfluid conduit 122 (FIG. 7). Pin 102 has pivotally mounted thereon a pairof depending links 160 which are vertically slidable through cylinders148 and which have outwardly flared passages 162 at their lower endsaligned with the bores in cylinders 148. The piston elements 152 areprovided with cam rollers 164 which are reciprocable with the pistons inpassages 162 under the action of fluid introduced into chamber 156. Therollers are a close fit in the smaller diameter of passages 162. In theposition of the parts shown in FIG. 6 the slop in the linkage providedby bushings 106 and 108 is eliminated by the presence of cams 164 whichare closely received in the smaller diameters of flared passages 162 andact directly against links 160 to transmit any motion of links 60 to pin102 without lost motion. When, however, the pistons are forced apart bythe fluid in chamber 156 the cams 164 are located in the flared portionof passages 162 and the link 60 can move relative to the pin 102, i.e.the slop eliminator is inoperative. Tension springs 165 connect pin 102with the two cylinders 148 and serve to support the weight of thesecylinders and link 60.

Electric sub-servornotor 144 is mounted directly on the casing of thepilot valve, as shown in FIG. 4, and has its rotatable shaft in the formof a jack screw 1 66 located alongside the pilot valve piston 116. Thejack screw has a traveling nut 168, as shown in FIG. 8, threaded thereoncarrying the pivot 142 which is inthe form of a trunnion pin received inthe bifurcated free end of lever 112. Thus as the jack screw is rotatedin one direction or the other the pin 142 moves up and down and movesthe lever 112 about pivot 110 at its right-hand end to move the pilotvalve 116. As shown, upper and lower adjustable stops 170 and 172 areprovided on the jack shaft in the form of threaded nuts which limit themovement of lever 112 by the electric sub-servomotor. It will be evidentin FIG. 4 that the lever 112 is a dilferen-tial lever connected at itsmidpoint to the pilot valve and at its opposite ends to link 104 andjack screws 166. As a result, movement of the pilot valve piston 116 canoccur either as a result of movement imparted to link 104 by the pilotor movement imparted to jack screw 166 or a simultaneous movement ofboth the link and the jack screw.

The sub-servomotor unit 144 as shown in detail in FIG. 8 includes anarmature 174 fixed on jack screw 166 and fields 176. Also on the jackscrew 166 is the armature 178 of a rate generator 179 having fields 180.The rotation of the jack screw by armature 174 generates a buckingvoltage in the rate generator to damp the movement of thesub-servomotor. Also in the same casing with the sub-servomotor andcoaxial therewith is a follow-up transformer 181 having windings 182 anda core 184 which is threaded onto the lower extended end of jack screw166 so that as the armature 174 rotates the core 184 is moved up or downto vary the voltage output of the transformer. The operation of thecontrol system thus far described will be explained in detailhereinafter.

The tail rotor pitch control is shown in FIGS. 9, 9a and wherein thepilots and copilots rudder pedals are shown at 186 and 188. Differentialmovement of either set of pedals results in swinging of a doublebellcrank 190 about a fixed vertical pivot 192 to in turn reciprocate arod .194. Rod 194 is attached to a projecting arm 197 which forms a partof and rotates a cable segment 196 about rod 198 upon which segment 196is supported and is freely rotatable to operate opposed reaches of cable200 the ends of which are connected at 202 to a lever 204. Lever 204 ispivoted intermediate its ends at 206 to a piston rod 207 of a primaryhydraulic servornotor 207w and at 208 to a rod 210 which is pivotallyconnected to the sloppy link 2111 (FIG. 12). A link 212 is pivoted tothe power piston at 206 and is pivoted at 214 to a cable sector 216movable about a fixed pivot 218. Sector 216 drives the usual cables 219and connected chain 220 for actuating the blade pitch control mechanism(not shown) on the tail rotor. An electric sub-servomotor 222 isprovided, similar to the motor 144 above described for the main rotorpitch control, having a threaded jack screw 236 which oscillates a lever228 about a pivot 230 at the end of a depending arm 232 on the sloppylink 211. A pilot valve 229 is pivoted to lever 228 intermediate itsends at 231 for controlling admission of fluid to the primary servomotor207a.

In addition to the structure above described, a stick canceler 271 isprovided installed as shown in FIG. 2, in order to offset in thedifferential system, used in pitch and roll control, the bucking effectof the autopilot when the pilot moves the control stick to achieve a newflight condition. The stick canceler provides. a voltage proportional tostick displacement which is inserted into the servo system input andwhich offsets the bucking voltage generated by the effected gyro as aresult of the corresponding fuselage attitude change. The stick canceleris shown diagrammatically in FIG. -17. Here it will be noted thatmovement of the stick 26 to effect movement of the swash plate alsooperates the canceler potentiometer 271 through torque shaft 272 and aconnecting link 274 to create a voltage proportional to the displacementof the stick. This voltage is inserted in the servo system input throughwires 276 to ofiset the bucking voltage generated by the sensingelements which is applied at 278 as a result of fuselage attitude changecaused by the movement of the stick by the pilot. The operation of thiscanceler is shown diagrammatically in FIGS. 18 and 19. In a steady stateof hovering, the gyros do not create a voltage and the stick is nearlyvertical. When the pilot desires to fly forward, for example, the stickmust be pushed forward. This would cause the ship to nose down and acorresponding voltage would be generated by the gyros. As the stickmoves forward, however, a potentiometer is unbalanced in proportion tothe stick displacement. The voltage created by this unbalancedpotentiometer is equal and opposite to that voltage created by the gyro,and hence, no error signal is received at the sub-servomotor. Thus, thepilot may change the flight attitude of the ship without having to buckthe voltage created by that change in attitude.

It is also necessary to provide some means for controlling the referencelevel of any of the controlled degrees of freedom. First, it must bepossible for the pilot to engage the autopilot in any flight attitudeand have the autopilot hold that condition. Second, it is also desiredthat the pilot be able to alter the reference setting to which thehelicopter is slaved without disengaging the autopilot.

In the pitch and roll systems which are straight differential systems,any change in reference setting can be accomplished by simplemanipulation of the stick. However, in the yaw system where the pilotspedals cannot have a steady state relationship to azimuth angle, someadditional means of changing the reference is required. To null pitchand roll reference signals, electric means for driving the autopilot tonull the motion of the azimuth stick are used so arranged that thehelicopter flight attitude is undisturbed in the procedure. These meansinclude an additional nulling potentiometer in the system which iscapable of generating a voltage to offset any possible combination ofsteady state voltages introduced by the sum of the sensing elements asdescribed more in detail hereinafter. An automatic self-tracking systemis used in the yaw system in which the autopilot automatically nullsitself before engagement, and also nulls itself any time the pilotoverrides the force exerted during open loop operation. Included in thisstructure is a panel instrument known as a heading selector by which thepilot may manually alter the autopilot azimuth setting when theautopilot is engaged. The direction or heading selector can be clutchedto a gyro compass. When the automatic pilot is in operation anydeviation between the two will generate a corrective voltage in thecontrol circuit to actuate the servomotor. Prior to engagement of theautopilot the units are clutched together so that upon engagement therewould be no error signal and the helicopter will hold the direction ofthat instant. The units are declutched upon engagement of the autopilot,and to fly in any direction the pilot may move the direction selector togenerate a corrective voltage until the compass is realigned with theselector.

The autopilot system also includes means for enabling the ship to hoverover a fixed point or to minimize drift when hovering over water. Thisstructure is shown in FIGS. 20, 21 and 22 and consists of a line 2 80with a weight 282 attached which is dropped either in the water or onland. This line passes through an aperture 284 formed by theintersecting slots in two right angularly disposed bales 286 and 287each of which has a slot 288. The bales are pivotally supported at theirends in a common horizontal plane on pivots 2% and 292, the axes ofwhich are at right angles to each other. The line 280 passes upward overa pulley 294 pivoted at 296 and is secured to the fuselage at 299. Itwill be evident that any movement of the ship either lateral orlongitudinal will cause swinging of bales 287 and 2 86, respectively.The bales are operatively connected to the movable elements 298 and 300of laterally and longitudinally responsive potentiometers which, asshown in FIGS. 21 and 22, vary the voltage in wires 306 and 308 whichsupply the subservomotors of the pitch and roll control systems.

Before considering the operation of the autopilot system in detail, itwill be helpful to consider the block diagram shown in FIG. 11 whichshows the relationship of the system components in the pitch controlsystem by which the ship may be controlled by the autopilot or the pilotin any desired combination. In general the basic autopilot system shownin FIG. 11 for pitch control comprises a position gyro 31 8, a rate gyro320 and a follow-up transformer 181. The two gyros contribute a signalvoltage which drives sub-servomotor 144. This motor drives a rategenerator 179 and the follow-up transformer 18 1 until the lattercontributes a voltage which alone offsets the signal voltage. Thesub-servomotor 144 through its jack screw 166 positions the hydraulicservo, or pilot valve, 116. The pilot valve controls the primaryservomotor which is connected to the pitch changing mechanism of therotor head. Also shown in FIG. 11 is the arrangement of the nullerpotentiometer, or synchro, 332 controlled by clutch 234 convenientlycontrolled from the pilot stick, and the canceler potentiometer, orsynchro, 271. It will be understood that in yaw control the canceler andnuller will be replaced with a heading selector, while in roll controlthe nuller potentiometer will be omitted.

The system can be operated in three ways (1) hydraulic power off,autopilot off. When the hydraulic power is ofl the automatic slopeliminator is operative giving a definite pivot point so that the trainof command is through the mechanical linkage only from the pilots stick,(2) hydraulic power on, autopilot off. In this operation the slopeliminator is released and normal servo operation from the stick takesplace. The train of command comes through the linkage from the pilotsstick to actuate the pilot valve, the primary hydraulic servomotor andhence the pitch changing mechanism, (3) hydraulic power on, autopiloton. Here the slop eliminator is held released, excitation comes fromvoltage from the gyros to the subservomotor which drives the jack screwand opens the pilot valve bringing the primary servomotor into action.At the same time a voltage is being created in the followup transformerwhich when equal and opposite to the gyro signal will cause the motor tocease turning, allowing the travel of the primary servomotor to closethe pilot valve in the usual follow-up operation.

In describing the operation of the autopilot system reference is madefirst to the diagrammatic showing of the pitch control mechanism in FIG.13, and to FIGS. 26 which have previously been described, showing thecorresponding mechanism.

As previously stated, there are three ways in which the system can beoperated, (l) with the hydraulic power off and the autopilot olf,movement of the pilots stick 26 forward (FIG. 2) results in a downwardmovement of link 56 associated with the left-hand servomotor unit 58resulting in downward movement of the link 60 associated with that unit.Since there is no pressure in chamber 158 of the slop eliminator, therollers 164 occupy the position shown in FIG. 6 and the piston 146 (FIG.13) is in a lowered position in which the passages 124 and 126 areconnected to provide free fluid communication between the chambers 128and on opposite sides of the primary servomotor piston 132. Under theseconditions movement of link 60 is conveyed without lost motion throughthe links directly to pin 102 and boss 44a on the relatively stationaryswash plate portion 40. This direct manual movement of the swash plateby the pilot stick is permitted because the servomotor cylinder 134 isfree to move relative to piston 132 by reason of the free communicationestablished by valve 146 above described. During this manual movement ofthe swash plate, the lever 112 is free to pivot about its pin 142 on thejack shaft 166.

(2) If the hydraulic power is on and the autopilot off, the slopeliminator will be released, i.e. the cam rollers 164 will be movedoutwardly in FIG. 6 due to the presence of hydraulic fluid in chamber156. With these cam rollers in the flared ends of passages 162 the link60 which carries the pistons 152 and the rollers 164 of the slopeliminator are free to move through the space afforded between bushings106 and 168 and to carry with it during this lost motion movement, thedepending link 104. This movement is sufficient to enable link 104 tomove lever 112 about its end pivoted at 142 to move the piston 116 ofthe pilot valve to the left, for example, in FIG. 13 thus admittinghydraulic fiuid under pressure in pipe 122 into passage 126 and chamber130 resulting in movement to the left of the cylinder 134 of the powerservomotor which is directly pivoted on pin 102. Movement of cylinder134 provides the usual follow-up for pilot valve 116. It will thus beevident that with the hydraulic power on and the autopilot off a normalservomotor operation from the pilots stick takes place, (3) with thehydraulic power on and the autopilot on, the slop eliminator is heldreleased. The manual servo operation described in (2) above is stillpossible. In addition, the sub-servomotor 144 is energized by voltagefrom the position gyro 318 as modified by the rate gyro 3 20 or from theother signal elements to correct any deviation from level flight. Forinstance, if the ship should for some reason start to nose down, avoltage is supplied to the sub-servomotor 144 to rotate it in adirection to cause the traveling nut 168 on the jack screw 166 to travelto the right in FIG. 13. Since lever 112 is pivoted at 110 to link 104which is at the moment stationary, the piston 116 of the pilot valvewill move to the right in FIG. 13 to admit hydraulic fluid throughpassage 124 to chamber 128. This will cause the main servomotor cylinder134 to move upwardly (FIG. 4) and to the right as viewed in FIG. 13 toraise boss 44a and correspondingly tilt the swash plate to bring theship back to level flight. During this operation of the motor 144 thelower threaded end of jack screw 166 has rotated a corresponding amountto move the core 184 (FIG. 8) threaded thereon sufficient to generate abucking voltage in the transformer which, as the ship returns to levelflight, will equal the voltage supplied to the motor 144. Since thevoltage supplied by the followup transformer 181 opposes the voltagefrom the position gyro which was supplied to the sub-servomotor 144, thelatter will cease to rotate when it has instituted a correction of thedown-nose tendency of the ship. Associated with the follow-uptransformer 144- is a rate generator 179 which has a damping effect onthe movements of the jack screw for the purpose of preventing overtravelof the sub-servomotor.

Also involved in the pitch control mechanism is a canceler 271 and anuller 332. The stick canceler described above is shown diagrammaticallyin FIG. 17 and its action is shown in FIGS. 18 and 19 which will bedescribed hereinafter.

In the yaw control mechanism shown diagrammatically in FIG. 12,additional means are required to incorporate the diiferential systemwhich will now be described. With hydraulic power on and the autopilotoff the slop eliminator piston 338 is held down by hydraulic pressure.Movement of the pilots stick rotates quadrant 204 about pivot 206 movingpoint 208. This movement is allowed by the slop eliminator 211. As point242 is fixed, point 231 will be forced to move, cracking the pilot valve229. This will release pressure to the primary servomotor 207a movingpoint 206 and also quadrant 216 about point 218 to change the tail rotorpitch. Since point 206 moves and the cables 200 are stationary, point208 will reverse its direction, moving point 231 and shutting oif thepilot valve 229.

Meanwhile, the heading selector 344 which contains a synchromotor slavedto the gyro compass has been keeping track of the ships heading. Whenthe autopilot is turned on the heading selector is locked to the trimknob and the ship is slaved to the compass. Thus any deviation from theheading selector course creates an error signal from the rate andposition gyros 646 and 348 which is amplified by the amplifier 350 anddrives the electric subservomotor 222 which is identical withsub-servomotor 144 described in connection with the pitch controlsystem.

As this motor turns, an opposing voltage is generated in the follow-uptransformer 181. This voltage will eventually equal the voltage from thegyros 346 and 348 and will eventually cause the motor 222 to stop. Thusfor a given gyro voltage, the motor will make only a certain number ofturns. The motor 222 drives the jack screw 166 which rotates arm 2 28about pivot point 208. This opens the pilot valve 229 permitting highpressure fiuid to act against power piston 207. This piston moves sector204 about fixed point 202, moving point 208 so that arm 228 assumes aposition such that point 231 returns to neutral and the pilot valve 229closes. The slop eliminator 33 8 is so constructed that for a relativelysmall heading error the travel of the jack screw 166 will be less thanthe free travel in the slop eliminator. Consequently, no feedback signalenters the pilots controls and the system is in closed loop.

However, in the case of a relatively large heading error, the followingoperation takes place: the jack screw 166 will move point 242 carried bynut 168 and drive arm 228 about point 208 opening the pilot valve 229 asbefore. High pressure fluid will move piston 207 and change the tailrotor pitch. Built into the slop eliminator plate 211 is a spring 352mounted between abutments in the plate. Plate 211 reciprocates spring352 in a plate having a notch 354 the opposite abutments of which arenormally spaced from the ends of spring 352. As piston 207 moves, sector204 will rotate about point 202 until the free movement between spring352 and its abutments is used up. The large signal turns the motor 222beyond the range corresponding to unrestricted movement of member 211.The pilot valve 340 will still be open and piston 207 will continue tomove. Consequently when plate 211 has moved sufficiently to bring spring352 into engagement with one or the other of the abutments of notch 354,point 208 instead of being restrained begins to compress the spring 352upon which it is bearing. Point 202 and the pedals begin to move. Thesystem is now in open loop. However, as the spring force is in the orderof ten pounds or so, the pilot may overcome the system by applyingpressure on the pedals. When this happens, the pilot applies force at apoint 202 to sector 204 causing it to rotate about point 206, thusfurther compressing the spring 352 at point 208. Ultimately plate 211will engage the slop eliminator piston 338 and point 208 will then besolidly restrained. The stops and 172 on jack screw 166 are such thatopen loop cannot be maintained when the spring 352 is compressed.Accordingly, arm 228 closes the pilot valve 340. Thus the yaw system isactually a force limited open loop, but may be considered a feelautopilot capable of exerting suflicient force to control the ship butwhich may be overcome by the pilot with a relatively small force.

The roll control mechanism illustrated in FIG. 14 consists of duplicatesystems which are identical with that used for the pitch control showndiagrammatically in FIG. 13. In these duplicate mechanisms the pivotpins 102 are connected respectively with bosses 44c and 4412 on therelatively stationary portion of the swash plate. These comprise theright and left lateral cyclic pitch control bosses as shown most clearlyin FIG. 2. It will be understood that movement of the pin 102 either bythe pilot or by the gyros will move the swash plate to provide lateralcyclic pitch control, the duplicate mechanisms connected to bosses 44cand 44b being differentially operated. The motors 144 are reversiblemotors and rotate in opposite directions for position signals. Thusmovement of the stick to the left will lower boss 44b and raise boss44c, while movement of the stick to the right will raise 44b and lower440. For altitude signal the motors 144 rotate in the same direction andare energized simultaneously with motor 144 of the pitch control system.

As previously stated, it is necessary to provide some means forcontrolling the reference level of any of the control systems abovedescribed. In the present autopilot system, the pilot is able to alterthe reference setting to which the helicopter is slaved Withoutdisengaging the autopilot. In the pitch and roll systems which arestraight differential systems, this change in reference setting isaccomplished by a simple manipulation of the pilots control. Anadditional nulling potentiometer 332 (FIG. 16) is provided capable ofgenerating a voltage to offset any possible combination of steady statevoltages introduced by the sum of the sensing elements 358. To eliminateadditional pilot controls, this potentiometer is installed so as toallow it to be operated by the pilots stick 26 when a clutch 234 isengaged by an actuating button 362 on the pilots stick. It is thenpossible to adjust the sensitivity of this potentiometer at 374 so thatany stick motion while the clutch 234 is engaged results in adisplacement of the autopilot sub-servomotor exactly equal and oppositeto the contribution to blade angle introduced by the straight mechanicallinkage. Thus to null the autopilot in the center of its operating rangethe pilot has only to push the button 362 and move the stick until anull indicator provided on the instrument panel indicates the servo iscentered. While doing this he dom not disturb the steady state operationof the helicopter since the blade angle has not been changed. Theautopilot displacement having been traded in elfect for pilots stickdisplacement with the clutch 234 disengaged, the voltage ofpotentiometer 332 is unchanged and remains as a bias on the rest of thesystem.

Referring to FIGS. 16 to 18, the stick 26 operates the potentiometer 332when the clutch 234 is engaged by the button 362 on the pilots stick.The stick 26 is also connected by a rod 364 to one end of a whiffietree366 at pivot point 368. This whifiletree represents the mixing linkageshown in greater detail in FIGS. 13 and 14. The sub-servomotor 144,driving the usual jack screw 166,

drives the nut which is pivotally connected at 142 to the other end ofthe whifiietree. The midpoint 370 of the whifiietree is pivotallyconnected by suitable linkage 372 to the non-rotating portion of theswash plate. Thus the differential action of the pilots signal from thestick 26 and the autopilots input is differentially applied. Motion ofthe pilots stick 26 with clutch 234 engaged causes the potentiometer 332to be unbalanced so as to impose a voltage on the autopilotsub-servomotor, and signal combination, whose magnitude in respect tothe amount of stick motion is controlled by the sensitivity adjustmentat 374. By proper control this voltage should be just sufficient for theresulting output of the sub-servomotor combination to offset themechanical input at the other end of the whiffietree 366 so that thecombined output of the differential system as applied to the controls isleft unchanged.

While the systems described hereinbefore provide pilot feel during thepartial range of travel in which the spring is engaged, a system whichutilizes a full feel is shown in FIG. 15. While these systems arebasically identical, here the spring 352 is immediately compressed uponmoving the slop eliminator slide 211, and a follow-up potentiometer 380is provided in the gyro circuit which is directly operated by the stick.Thus, when an error signal occurs while the stick is held stationary,the servo motor 222 introduces a correction limited by the follow-uptransformer 181. Since this correction results in a proportionalcompression of the spring 352, a force proportional to the correction isfelt in the stick 26. If the stick is allowed to move, the mechanicalmotion of the stick moves the follow-up potentiometer 386 untilsufficient voltage is introduced to allow the motor to return to aposition where the spring 352 is centered and no force is present in thestick because of the original error signal. With this arrangement it ispossible to introduce corrections to the control surface with forceslimited only by the capacity of the hydraulic power piston, at the sametime limiting the forces which move the pilots stick to an amountindependent of the power piston and equal to the force output determinedby the spring 352.

In FIGS. 23 and 24 the electrical diagrams of the pitch and yaw systemsare shown. In these diagrams the numerals correspond to those used inthe pitch and yaw systems shown in the mechanical and schematic views.The switches 385 shown leading from the compass circuit can be manuallyactuated switches, such as knife or micro switches, and they may bemounted in any convenient place, such as on the control panel or theactual rudder pedals so that when it is desired to change heading thecompass is reconnected by closing the switches. This is obviouslynecessary in the yaw system where big changes in angles of heading up toone hundred eighty degrees may be desired.

It will be evident that as a result of this invention an autopilot hasbeen provided which utilizes the primary hydraulic servo motors as theactuating members in the autopilot system. It will further be evidentthat an autopilot has been provided which utilizes the same controls forthe autopilot as are normally used for the pilots manual control byproviding an electric sub-servomotor within the primary hydraulic powercontrol to produce a differential control correction at the output powerlevel of the servo for a differential autopilot system.

In addition, an automatic fail safe provision is made to lock out theautopilot differential input in the event of a failure of the primaryhydraulic system. Also, in the open loop tail rotor control, means hasbeen provided to utilize a low power sub-servomotor in the autopilotsystem to exert considerably amplified forces on the pilots pedals, orthe pilots stick, to produce a stick free autopilot system which thepilot can overpower and manipulate the helicopter directly.

it will further be evident that improved canceler and nulling deviceshave been provided for an autopilot sysf2 tem whereby improved resultsare obtained in the control of the helicopter.

While one embodiment of the improved autopilot system has been shown byway of illustration, it will be understood that many changes in theconstruction and arrangement of the parts are contemplated within thescope of the invention.

We claim:

1. An aircraft having in combination, a rotor head having a blademounted for pitch changing movement, a primary servomotor connected tosaid blade for changing the pitch of said blade, a pilot operativemember for controlling said servomotor, a sub-servomotor for controliingsaid servomotor, aircraft attitude responsive means for controlling saidsub-servomotor, and means for differentially applying the control forcesfrom said pilot operative member and said sub-servomotor to said primaryservomotor.

2. In an aircraft in combination, a movable surface for controlling anaircraft, a servomotor connected to said surface for moving saidsurface, valve means for actuating said servomotor, a pilot operativemember, reference means on the aircraft to be controlled for producing asignal in response to a departure from a predetermined position of saidaircraft, a sub-servomotor actuated by said signal, first meansoperatively connecting said pilot operative member and saidsub-servomotor to said valve means whereby either said member or saidsub-servomotor or both together may move said valve means, and secondmeans also connecting said pilot operative member to said surface fordirect movement of said surface by said pilot operative member, saidlast named second means including a device for disabling said secondconnecting means so that said pilot operative member moves said valvemeans without directly moving said surface.

3. In an aircraft in combination, a movable surface for controlling anaircraft, means for actuating said surface, a servomotor connected tosaid actuating means for moving said surface, valve means for actuatingsaid servomotor, a pilot operative member, reference means on theaircraft to be controlled for producing a signal in response to adeparture from a predetermined position of said aircraft, asub-servomotor actuated by said signal, first means operativelyconnecting said pilot operative member and said sub-servomotor to saidvalve means whereby either said member or said sub-servomotor or bothtogether may move said valve means, and second means also connectingsaid pilot operative member to said actuating means for direct movementof said means by said pilot operative member, said last named secondmeans including a device for disabling said second connecting means sothat said pilot operative member moves said valve means without directlymoving said actuating means.

4. In an aircraft in combination, a movable surface for controlling anaircraft, means for actuating said surface, a servomotor connected tosaid actuating means for moving said surface, first valve means foractuating said servomotor, a pilot operative member, reference means onthe aircraft to be controlled for producing a signal in response to adeparture from a predetermined position of said aircraft, asub-servomotor actuated by said signal, first means operativelyconnecting said pilot operative member and said sub-servomotor to saidfirst valve means whereby either said member or said sub-servomotor orboth together may move said first valve means, and second means alsoconnecting said pilot operative member to said actuating means fordirect movement of said means by said pilot operative member, saidsecond connecting means including second valve means providing for freemovement of said servomotor, said second connecting means including adevice for disabling said second connecting means so that said pilotoperative member moves said first valve means without directly movingsaid actuating means.

5. In an aircraft in combination, a movable surface for controlling anaircraft, means for actuating said surface, a servomotor connected tosaid actuating means for moving said surface, first valve means foractuating said servomotor, a pilot operative member, reference means onthe aircraft to be controlled for producing a signal in response to adeparture from a predetermined position of said aircraft, asub-servomotor actuated by said signal, first means operativelyconnecting said pilot operative member and said sub-servomotor to saidfirst valve means whereby either said member or said sub-servomotor orboth together may move said first valve means, and second means alsoconnecting said pilot operative member to said actuating means fordirect movement of said means by said pilot operative member, saidsecond'connecting means including second valve means providing for freemovement of said servomotor, said second connecting means including adevice for disabling said second connecting means so that said pilotoperative member moves said first valve means without directly movingsaid actuating means, said disabling device being connected to saidsecond valve means so that when said second connecting means is disabledsaid servomotor is not provided free movement by said second valvemeans.

6. A helicopter having in combination, variable pitch blades and controlmeans connected with said blades for controlling the pitch of thelatter, a hydraulic servomotor operatively connected to said controlmeans for changing the pitch of said blades, said servomotor having apilot valve for admitting hydraulic fluid thereto, a pilot operativemember, lost motion connection means connecting said member with saidcontrol means, means on the helicopter for generating a signal in.response to a departure from a predetermined position of saidhelicopter, a sub-servomotor actuated by said signal, and a differentiallinkage operatively connected with said pilot valve and with both saidsub-servomotor and said pilot operative member for controlling saidservomotor in response to either or both the movement of said generatingmeans or the movement of said pilot operative member permitted by saidlost motion connection, and means for preventing the lost motion in saidconnection means enabling said member to move said control meansdirectly.

7. In an aircraft in combination, a movable surface for controlling anaircraft, means for actuating said surface, a servomotor connected tosaid actuating means for moving said surface, first valve means foractuating said servomotor, a pilot operative member, reference means onthe aircraft to be controlled for producing a signal in response to adeparture from a predetermined position of said aircraft, asub-servomotor actuated by said signal, means operatively connectingsaid pilot operative member and said sub-servomotor to said first valvemeans whereby either said member or said sub-servomotor or both togethermay move said first valve means, means connecting said pilot operativemember to said surface to be maintained inoperative during operation ofsaid surface by said servomotor, and means for making said last namedconnecting means operative to move said surface by said pilot operativemember independently of a driving action by the servomotor. 1

8. A helicopter having in combination, variable pitch blades and controlmeans connected with said blades for con-trolling the pitch of thelatter, a hydraulic servomotor operatively connected to said controlmeans, said servo motor having a valve for admitting hydraulic fluidthereto, a pilot operative member for moving said valve, means on thehelicopter for generating a signal in response to a departure from apredetermined position of said helicopter, a sub-servomotor actuated bysaid signal, a differential linkage operatively connected directly withsaid valve and with both said sub-servomotor and said pilot operativemember for controlling said servomotor, and means operatively connectingsaid pilot operative member with said control means for moving saidblade independently of said valve.

9. A helicopter having in combination, variable pitch blades and controlmeans connected with said blades for controlling the pitch of thelatter, a hydraulic servomotor operatively connected to said controlmeans for changing the pitch of said blades, said servomotor havingvalve means for admitting hydraulic fluid thereto, a pilot operativemember for moving said valve means, lost motion connection meansconnecting said pilot operative member with said control means enablingsaid member to move said valve means through an initial movement of saidmember without moving said control means directly, means on thehelicopter for generatinga signal in response to a departure from apredetermined position of said helicopter, a sub-servomotor actuated bysaid signal, a differential linkage operatively connected with saidvalve means and with both said sub-servomotor and said pilot operativemember for controlling said servomotor in response to operation ofeither or both of said pilot operative member and said generating means,and a source of hydraulic fluid under pressure, said lost motionconnection means having means for eliminating said lost motion in theconnection between said pilot operative member and said control means sothat said pilot operative member can move said control means directly.

10. A helicopter having in combination, variable pitch blades andcontrol means connected with said blades for controlling the pitch ofthe latter, a hydraulic servomotor operatively connected to said controlmeans for changing the pitch of said blades, said servomotor havingvalve means for admitting hydraulic fluid thereto, a pilot operativemember for moving said valve means, lost motion connection meansconnecting said pilot operative member with said control means enablingsaid member to move said valve means through an initial movement of saidmember without moving said control means directly, means on thehelicopter for generating a signal in response to a departure from apredetermined position of said helicopter, a sub-servomotor actuated bysaid signal, a differential linkage operatively connected with saidvalve means and with both said sub-servomotor and said pilot operativemember for controlling said servomotor in response to operation ofeither or both of said pilot operative member and said generating means,and a source of hydraulic fluid under pressure, said lost motionconnection means having means for eliminating said lost motion in theconnect-ion between said pilot operative member and said control meansincluding cam means movable into a position to eliminate said 10stmotion whereby said pilot operative member can move said control meansdirectly.

11. A helicopter having in combination, variable pitch blades andcontrol means connected with said blades for controlling the pitch ofthe latter, a hydraulic servomotor operatively connected :to saidcontrol means for changing the pitch of said blades including a cylinderhaving a piston therein, said piston forming chambers on opposite sidesthereof with said cylinder, said servomotor having first valve means foradmitting hydraulic fluid thereto, a pilot operative member for movingsaid first valve means, lost motion connection means connecting saidpilot operative member with said control means enabling said member tomove said first valve means through an initial movement of said memberWithout moving said control means directly, means on the helicopter forgenerating a signal in response to a departure from a predeterminedposition of said helicopter, a subservomotor actuated by said signal, adifferential linkage operatively connected with said first valve meansand with both said sub-servomotor and said pilot operative member forcontrolling said servomotor in response to operation of either or bothof said pilot operative member and said signal generating means, and asource of hydraulic fluid under pressure, said lost motion connectionmeans having means for eliminating said lost motion in the connectionbetween said pilot operative member and said control means so that saidpilot operative member can move said control means directly and secondvalve means for connecting the chambers on opposite sides of said pistonof said servomotor when said lost motion has been eliminated.

12. In an aircraft in combination, a movable surface for controlling anaircraft, primary hydraulic servomotor means for moving said surfaceincluding a pilot valve, pilot operative means for controlling saidpilot valve, gyroscopic means for controlling said pilot valve, anddifferential mechanism operatively connected with both of said pilotoperative means and gyroscopic means and with said pilot valve foroperating the latter upon operation of either or both of said means toenergize said servomotor means, said pilot operative means also havingmeans operatively connecting it with said surface for moving saidsurface independently of said pilot valve.

13. In an aircraft in combination, a movable surface for controlling anaircraft, a primary servomotor operatively connected to said surface formoving said surface, valve means operatively connected with said primaryservomotor for actuating said primary servomotor, a differentialactuating link, a pilot operative member for operating said valve meansbeing connected to said link adjacent one end of said link, said pilotoperative member also having a lost motion connection with said movablesurface for operating the surface directly, a subservomotor foroperating said valve means being connected to said link, said link beingoperatively connected to said valve means for actuating said valvemeans, and means for generating a signal in response to a departure froma predetermined position of said aircraft, said subservomotor beingresponsive to said signal.

14. In an aircraft in combination, a movable surface for controlling anaircraft, a servomotor having cylinder and piston units, said servomotorbeing operatively connected to said surface for moving said surface,first valve means operatively connected with said servomotor foractuating said servomotor, a differential actuating link, said linkbeing operatively connected to said first valve means for actuating saidfirst valve means, a pilot operative member for operating said firstvalve means being connected to said link, a sub-servomotor for operatingsaid first valve means being connected to said link, and means forgenerating a signal in response to a departure from a predeterminedposition of said aircraft, said subservomotor being responsive to saidsignal, said pilot operative member also having means operativelyconnecting it with said surface for moving said surface independently ofsaid first valve means, said connecting means including second valvemeans for connecting one end of said cylinder unit to the other toprovide a passageway therebetween, said connecting means being renderedinoperative by a device which directs movement of said pilot operativemember to move said first valve means.

15. A helicopter having in combination, a rotor having blades mountedfor pitch changing movements, means for controlling the pitch of saidblades cyclically including rotatable and non-rotatable swash platemembers, a pilot opeartive control member, hydraulic servomotormechanism providing power operation of said blade pitch control meansincluding a servomotor having a cylinder and piston elements, one ofwhich is connected to fixed structure of the helicopter and the other ofwhich has an operative connection to said non-rotatable swash plate, asource of hydraulic fluid under pressure, a pilot valve for meteringfluid to opposite sides of said piston having an operative connection tosaid pilot operative control member, lost motion means connecting saidpilot operative member and said non-rotatable swash plate memberpermitting an initial movement of said pilot operative control memberand said pilot valve free from said pitch control means sufficient tometer fluid to said servomotor,

and means for controlling said pilot valve independently of the controlthereof by said pilot operative member including a sub-servomotoroperatively connected with said pilot valve and a position responsivedevice on the helicopter for generating a signal to energize saidsubservomotor in response to a departure from a predetermined positionof said helicopter.

16. A combination as claimed in claim 15 in which said lost motion meansincludes means providing a direct connection between said pilotoperative member and said non-rotatable swash plate member and forestablishing fluid communication between opposite sides of saidservomotor piston in response to failure of hydraulic pressure in saidsource.

17. In an aircraft in combination, a movable surface for controlling anaircraft, a primary servomotor for moving said surface, valve means foractuating said primary servomotor, a pilot operative member foroperating said valve means, a sub-servomotor for operating said valvemeans, means for generating a signal in response to a departure from apredetermined position of said aircraft, said sub-servomotor beingresponsive to said signal, and differential operating means operativelyconneoting said pilot operative member and said sub-servomotor to saidvalve means, said pilot operative member being also connected to saidmovable surface through another connection for directly controlling saidsurface.

18. In combination in a helicopter of the type having a main sustainingrotor with variable pitch blades, a pilot operative member havingcontrol means for varying the pitch of said rotor blades to control saidhelicopter including a primary servo and a pilot valve for admittinghydraulic fluid to operate said servo, a sub-servomotor also operativelyconnected with said control means to operate said pilot valve, gyromeans for energizing said sub-servomotor in response to a movement ofsaid helicopter, a lost motion connection between said member and saidcontrol means for permitting an initial movement of said valve by saidmember to admit hydraulic fluid to said primary servomotor, a source ofpressure fluid, and means to limit the movement of said lost motionconnection upon large movements of said control means by saidsub-servomotor.

19. An aircraft having in combination, a movable surface operablyconnected to said aircraft for controlling an aircraft, a primaryservomotor operably connected to said surface for moving said surface, apilot operative member for controlling said servomotor, a sub-servomotorfor controling said servomotor, a position responsive gyro operablyconnected to said sub-servomotor for energizing said sub-servomotor,means operably connected to said pilot operative member, saidsub-servomotor, and said primary servomotor for differentially applyingthe control forces from said pilot operative member and saidsub-servomotor to said primary servomotor, and means operably connectedto said pilot operative member enabling the pilot to override said gyroenergizing force to alter the reference heading of the aircraftincluding means for opposing the gyro signal during movement of saidpilot operative member.

20. An aircraft having in combination, a movable surface for controllingan aircraft, a primary servomotor for moving said surface, a pilotoperative member for controlling said servo motor, a sub-servomotor, aposition responsive gyro for energizing said sub-servomotor, and meansfor differentially applying the control forces from said pilot operativemember and said sub-servomotor to said surface, and a potentiometerconnected to said pilot operative member capable of generating a voltageto offset any voltage introduced by said gyro due to movement of saidpilot operative member.

21. An aircraft having in an operative combination, a movable surfacefor controlling an aircraft, a primary servomotor for moving saidsurface, a pilot operative member for controlling said servomotor, asub-servomotor

